Cassini’s Final Mission: Obliteration

The Cassini spacecraft is dancing toward death — and NASA wants to make sure it doesn’t take any alien life with it.

Since 2004 the probe has twirled around Saturn, studying the gas giant’s rings, storms and moons. But it has recently started preparing for next year, when it will plunge into the planet’s atmosphere and vaporize.

NASA chose the ringed planet as Cassini’s final resting place because the space agency doesn’t want to risk contaminating a potentially habitable world with hardy microbes that may be aboard the craft. Saturn, with its gaseous surface consumed by hydrogen and helium, is inhospitable to life.

The same may not be true for some of its moons.

Titan, the biggest Saturnian satellite, has an atmosphere that is much less hostile than its overlord’s. It is a wet world similar to Earth, but unlike our planet, it is awash in methane.

“Methane plays the role on Titan that water plays on Earth,” said Linda Spilker, the Cassini project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Methane can be a vapor and form clouds. It can rain methane and that methane can flow into tiny lakes and large seas at the north pole of Titan.”

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A composite image of Saturn’s moon Titan taken by the Cassini spacecraft.CreditNASA

Cassini is currently exploring the methane seas and trying to determine if they have waves. Its goal is to detect mirror-like reflections of sunlight off the liquid surface, known as specular reflections.

“Someday we might land a boat on the sea of Titan and make measurements,” she said. “You could have very unusual kinds of life in those seas, life that uses methane instead of water.”

To protect Titan and other potentially life-bearing places, Cassini’s final mission calls for its obliteration. Researchers have dubbed the spacecraft’s swan song the “Cassini Grand Finale” because it includes an ambitious maneuver: 22 loops through the gap between Saturn’s surface and its innermost rings.

Before launching Cassini, NASA drew up plans to dispose of it in Saturn’s upper atmosphere, but they only developed its elaborate end-of-mission maneuver in the past couple of years. Once they found that some of Saturn’s moons may be potentially habitable to life, keeping the probe clear of them as it ran out of fuel became crucial.

Before Cassini can thread the needle through the planet’s gap, it must first reach the proper angle in its orbit. That means climbing to an inclination of approximately 64 degrees. The problem is that Cassini doesn’t have the power to propel itself to that position.

“The fuel tank gauge is on ‘E,’” said Dr. Spilker, adding that the probe is in desperate need of a gravity boost. To get one, it will tango with Titan.

“Imagine Titan as having a giant extra fuel tank,” Dr. Spilker said. By performing a flyby of the moon, Cassini can swing from one partner to another. “We can use its energy and gravity to raise the tilt of Cassini’s orbit.”

Earlier this month Cassini completed a flyby that raised its inclination from 35 degrees to 42 degrees. It has plans to conduct five more this year before its final mission.

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An artist’s rendering of Cassini’s final mission, a series of 22 loops over a few months.CreditNASA

“Each of the six will raise the tilt of the orbit a little bit,” said Dr. Spilker, “until we get the right orbit to go in between the rings and the planet.”